1. Technical Field
The present invention relates to a high electron mobility transistor (HEMT) formed from a gallium nitride (GaN) material system and more particularly to GaN based HEMT transistor with reduced leakage current.
2. Description of the Prior Art
High electron mobility transistors (HEMTs) are well known in the art and are known to be used are used in various low noise and power microwave applications where high device output power, power added efficiency and noise performance are critical. HEMTs are known to be used in Q, V and W band microwave power amplifiers in commercial and military radar systems, communications systems and the like. HEMTs are also known to be effectively integrated into monolithic microwave integrated circuits and monolithic millimeter wave integrated circuits (MMICs) including phased arrays for radiating at high power levels.
HEMTs formed from Gallium Nitride (GaN) material systems are known. Examples of such GaN based HEMTs are disclosed in U.S. Pat. Nos. 6,064,082; 6,534,801; 6,461,944; U.S. Patent Application publications: US 2002/0079508; US 2002/0096692; US 2002/0106842; US 2002/0167023; and US 2003/0020092. Such GaN HEMTs are also disclosed in Japanese patent no. JP2002280571 as well as International application nos.: PCT/US02/23056; and PCT/US98/04218. Such GaN HEMTs are well published in the literature. See for example: “HIGH-POWER MICROWAVE GaN/AlGaN HEMT's ON SEMI-INSULATING SILICON CARBIDE SUBSTRATES”, by S. T. Shepard et. al, IEEE Electron Device Letters, Vol. 20, No. 4, April 1999, pages 161-163; “Application of GaN-Based Heterojunction FETs for Advanced Wireless Communication”, by Yasuo Ohno et. al. IEEE Transactions on Electron Devices, Vol. 48, No. 3, March 2001, pages 517-523; “Galium Nitride Based High Power Heterojunction Field Effect Transistors: Process Development and Present Status at UCSB”, by Stacia Keller et. al., IEEE Transactions on Electron Devices, Vol. 48, No. 3, March 2001, pages 552-559; “Technology and Reliability Constrained Future Copper Interconnects—Part I: Resistance Modeling”, by Pawan Kapur et. al., IEEE Transactions on Electron Devices, Vol. 49, No. 4, April 2002, pages 590-597; and “Copper Gate AlGaN/GaN HEMT with Low Gate Leakage Current”, by Jin-Ping Ao et. al., IEEE Electron Device Letters, Vol. 24, No. 8, August 2003, pages 500-502.
Such GaN based HEMT devices are known to have relatively high gate leakage current. Different techniques have been tried to reduce the leakage current of such GaN based HEMTs. For example, as discussed in “Copper Gate AlGaN/GaN HEMT With Low Gate Leakage Current”, supra, leakage currents were shown to be reduced based on the type of metal used for the gate contacts. In particular, copper (Cu) gate contacts were demonstrated to provide reduced gate leakage current relative to comparable devices with gates formed from Ni/Au.
Other techniques for reducing the leakage current of a GaN based HEMT are disclosed in U.S. Patent Application Publication No. U.S. 2003/0020092 A1, published on Jan. 30, 2003, and International Publication No. WO 03/032397 A2 (International Application No. PCT/US02/23056). These publications disclose a GaN based HEMT formed with an insulated gate. In particular, the HEMT is formed with a A1xGa1-x buffer layer on top of a substrate. A high resistivity layer is formed on top of the buffer layer. A barrier layer, formed from AlGaN, is formed on top of the high resistivity layer. Drain and source contacts are formed on top of high resistivity GaN layer. An insulating layer, formed from SiN, is formed on top of the AlGaN barrier layer. The gate contact is formed on top of the insulating layer. Since the gate contact is insulated from the drain and source contacts, the gate leakage current is reduced. While the above technique may work fairly well with GaN/AlGaN HEMT layered devices, this technique is not applicable to HEMT devices formed without an insulated gate from GaN based materials.
FIGS. 1A-1C illustrate the problem associated with such devices. In particular, the process begins, for example, with a piece of GaN based material 20. As shown in FIG. 1B, the GaN based material is etched to form a mesa region 22 which defines step discontinuities 24 and 26 for the source and drain regions, respectively. As shown in FIG. 1B, these step discontinuities 24 and 26 allow undesirable leakage currents to flow between the source and gate and between drain and gate, resulting in relatively poor electrical isolation between the source and gate as well as between the drain and gate regions.
In addition to the problem mentioned above, the deposition of the gate metal 28 is known to form an overhang over the step discontinuity regions 24 and 26, as shown in FIG. 1C. In such a configuration, the cross sectional area of the gate metal over the step continuity may not be uniform, resulting in relatively thinner areas of gate metal resulting in a current constriction. Thus, there is a need for an HEMT device formed from GaN material with reduced leakage current, which also eliminates current constrictions resulting from deposition of the gate metal over discontinuities in the device.